It is known to provide an imaging device having a radiation detector in the form of a two dimensional array of pixels or pixel elements, each pixel element having a radiation sensor element for detecting radiation incident thereon. An image of a subject may be obtained by the device by projecting radiation onto the detector and determining the relative amounts of radiation incident upon each pixel element over a prescribed time period.
Known detectors include CMOS (complementary metal oxide semiconductor) detectors, CCD (charge coupled device) detectors, image intensifiers and the like. CMOS and CCD detectors are commonly used in domestic hand-held electronic devices such as mobile telephones and video cameras.
Such devices also find application in scientific instrumentation apparatus such as medical imaging systems, electron microscopes including transmission electron microscopes, medical and biological imaging applications, space imaging applications and security applications.
FIG. 1 is a schematic circuit diagram of a known CMOS active pixel element 100 also referred to as a 3T (three transistor) active pixel. The pixel element 100 has a radiation sensor element in the form of a photodiode 110 and three MOS transistors: a reset transistor 121, a source-follower input transistor 131 and a selection (‘select’) transistor 141. Source-follower input transistor 131 forms a source follower arrangement with current bias portion 150 which comprises a current mirror arrangement. The source-follower input transistor 131 may also be referred to as a source follower transistor 131.
Each of the transistors 121, 131, 141 has a source, a gate and a drain terminal.
The source of the reset transistor 121 is connected to a node X whilst the drain is connected to a supply of potential VRST. The gate of the reset transistor 121 is connected to a reset signal line RST. The gate, source and drain of the source-follower input transistor 131 are respectively connected to the node X, the drain of the selection transistor 141 and a supply of reference voltage VDD. The gate of the selection transistor 141 is connected to a row select line ROW and the source of the selection transistor 141 is connected to a column readout line COL.
It is to be understood that the source-follower input transistor 131 is arranged to act as a buffer of the signal applied to the gate thereof. When the current flow through the source-follower input transistor 131 is kept constant by an appropriate bias applied by the current bias portion 150 then, neglecting the second order effect of the activated selection transistor 141, the output voltage on the column readout line COL at terminal T is proportional to the potential applied to the gate of the source-follower input transistor 131 but with a much lower equivalent output impedance.
In operation, reset signal line RST is set HIGH (i.e. assumes a logical 1 condition) causing the reset transistor 121 to turn ON (i.e. the channel of the reset transistor 121 becomes conducting) and a potential Vx of the floating node ‘X’ is set to VRST. When the potential at Vx is set to VRST, the photodiode 110 stores charge therein due to the node capacitance of the photodiode 110, a region of space charge associated with the photodiode 110 being increased.
The reset signal line RST is then set LOW (i.e. controlled to assume a logical 0 condition) causing reset transistor 121 to turn OFF.
Radiation incident on the diode 110 is converted to mobile electron-hole pairs within the diode 110 causing a current to flow through the diode, discharging the charge accumulated by the photodiode 110 when the reset signal was applied. This in turn causes a change in Vx.
When it is required to read out Vx the row of the pixel 100 of FIG. 1 is selected by turning ON the selection transistor 141 (i.e. row line ROW is set HIGH). A signal corresponding to Vx is then applied by the selection transistor 141 to the column line COL which may also be referred to as an output line OUTP. The column line COL is in turn connected to signal processing electronics which reads out the potential at an output terminal T of the column line COL.
Periodically, reset signal line RST is set HIGH, connecting floating node X to VRST via reset transistor 121 and refreshing the amount of charge stored by the photodiode 110 due to its node capacitance. The potential at node X is thereby reset to VRST.
Applying VRST to the diode 110 biases the diode 110 and therefore VRST may be referred to as a bias voltage. In the embodiment of FIG. 1 VRST is arranged to reverse bias the diode 110 to increase the width of the depletion layer and improve detection response time. Furthermore it should be noted that the higher the reset voltage the more charge may be collected by the photodiode 110 before saturation.
It is to be understood that in the arrangement of FIG. 1 VX is monotonically dependant on the cumulative number of photo-generated electrons collected by the diode 110, which is in turn typically monotonically dependant on the level of illumination, specifically the illuminance (the total incident luminous flux, per unit area).
When the amount of accumulated charge at the diode 110 falls to a sufficiently low value VX ceases to change with further illumination and the diode 110 may be considered to be ‘saturated’.
It is to be understood that the amount of charge passed by the diode before reaching saturation depends on the node capacitance of the diode 110. The larger the node capacitance, the more charge can be passed by the diode 110 before saturation conditions are reached, and the greater the dynamic range of the pixel element 100. However, increasing the node capacitance causes an increase in the sampling noise (reset noise on node X) in the output signal of the pixel element 100 (i.e. the total potential read out at column line COL) reducing the signal to noise ratio (SNR). Therefore, in the device shown in FIG. 1 there is a trade-off between dynamic range and noise.
It is desirable to improve the dynamic range of pixels of radiation detectors to reduce a risk of saturation of the pixels 100 under high intensity illumination conditions without reducing the SNR.
Furthermore it is also desirable to reduce a problem of image gradient effects due to a drop in potential across output signal lines of pixel element arrays.
It is also desirable to enhance operational functionality of a radiation detector comprising a pixel element array.
Embodiments of the invention endeavour to mitigate at least one of the disadvantages of known radiation detectors.